|Sigma 20-40mm f2.8 EX
Aspherical DG DF Lens
by Jim McGee
When we were comparing the Nikon D100 and Sigma SD-9 digital cameras the folks at Sigma were kind enough to provide us with their new 20-40mm f2.8 EX Aspherical DG DF Lens in both Nikon and Sigma mounts so we could do a side-by-side comparison of the cameras. It was also a good opportunity to get to know the lens.
The 20-40mm is part of Sigma's EX pro series. That's obvious when you pick the lens up. It's no lightweight at 1.3 pounds and the weight is nicely balanced when mounted on a pro-camera, though it might feel a touch front-heavy on a light weight SLR.
The lens has a handsome exterior coated in black with gold trim. Zoom and focus rings are coated with a rubber grip and move with a well-damped feel. The aperture ring on the Nikon F-mount version moved with a smooth click stop from aperture to aperture.
This lens is targeted toward digital camera users. It's 20-40m range translates to 30-60mm on the D100 and 34-68mm on the SD-9 or about the range of a "normal" zoom, albeit a normal zoom with a fast constant f2.8 maximum aperture. The filter diameter on this lens, as with many of Sigma's newer lenses is 82mm. As more of these lenses get out in the field camera shops will stock filters in this size. But at present it's still unusual enough that you're not likely to find filters on the shelf and you may pay a premium for a special order. A lens hood is included.
Good news for you landscape shooters is that this is an internal focus lens so you won't have to worry about the front element rotating with when you focus or zoom.
In the Field
Autofocus speed was quick if not earth shattering; but fitting this lens with a hypersonic motor (HSM) would increase the price without really improving real world performance for most shooters. This is after all, not a focal length you'd normally use for shooting action.
Rectilinear distortion is the bane of all super-wide lenses. While there was some with this lens is was less noticeable than with Sigma's 17-35mm f2.8-4 EX. When shooting straight line subjects at 20mm some curvature of straight lines could be seen at the edges of the image. That distortion was gone by about 24mm and wasn't noticeable when the lens was used with a digital camera. Remember digitals only use the center of the image provided by the lens - so the camera clips off the areas where distortion occurred at 20mm anyway. In landscapes or scenes without obvious straight lines distortions shouldn't be noticeable at all.
I also wound up giving the lens an impromptu torture test. For several days hiking in Zion National Park heavy wet snow storms rolled over us on a regular basis. Temperatures changed with altitude and with storm fronts. Everything was wet and the fine red sand that makes up the Navajo sandstone cliffs got into everything. Both Sigma 20-40mm lenses performed flawlessly in these ugly conditions.
It's currently available for Sigma AF, Canon AF, Nikon AF-D, Minolta AF D and Pentax AF mounts.